1. Field of the Invention
The present invention relates to a semiconductor pressure sensor and a method of manufacturing a semiconductor pressure sensor, and more particularly to a semiconductor pressure sensor having a gauge resistor and a method of manufacturing a semiconductor pressure sensor.
2. Description of the Background Art
Conventionally proposed is a semiconductor pressure sensor including a diaphragm and a reference pressure chamber formed by bonding a flat plate-like silicon substrate and a silicon substrate having a concave portion.
For example, Japanese Patent Laying-Open No. 2000-124466 proposes a semiconductor pressure sensor including a diaphragm and a reference pressure chamber. In the semiconductor pressure sensor described in this publication, a first silicon substrate in the shape of a flat plate and a second silicon substrate having a concave portion are bonded with an oxide film formed on a surface of the first silicon substrate being interposed therebetween. The concave portion is sealed by the first silicon substrate to form a reference pressure chamber. Then, the second silicon substrate is ground to form a diaphragm at a portion that covers the concave portion of the second silicon substrate. Gauge resistors are formed at prescribed positions of the diaphragm.
In one method of manufacturing a semiconductor pressure sensor as described in the publication above, an alignment mark (first alignment mark) is formed simultaneously with the concave portion at a first surface of the second silicon substrate that is to be bonded to the first silicon substrate. Registration is performed with respect to the first alignment mark using an IR (infrared) aligner, so that an alignment mark concave portion (second alignment mark) is formed at a second surface opposite to the first surface of the second silicon substrate. Registration is performed with respect to the second alignment mark using a stepper, so that gauge resistors are formed at prescribed positions of the diaphragm.
In another manufacturing method described in the publication above, a first alignment mark is formed to pass through the second silicon substrate. Registration is performed with respect to the first alignment mark using a stepper, so that a second alignment mark is formed at the second surface of the second silicon substrate. Gauge resistors are thereafter formed in the same way as the one manufacturing method above.
In one method of manufacturing a semiconductor pressure sensor described in the publication above, the concave portion cannot be visually identified because the concave portion is shielded by the first and second silicon substrates. Therefore, the accurate position of the diaphragm covering the concave portion cannot be visually identified, either. Then, gauge resistors are formed at prescribed positions of the diaphragm using an alignment mark. An IR aligner is used to recognize the first alignment mark since the first alignment mark cannot be visually identified. A stepper having an IR alignment function is used as the IR aligner. However, steppers having the IR alignment function are extremely scarce. Even in the environment in which a stepper having the IR alignment function is available, the frequency of use of the IR alignment function is generally very low. Therefore, capital investment efficiency is decreased and the unit cost per process is increased. Accordingly, the production cost is increased.
In order to match the thickness of the diaphragm with the desired thickness, it is necessary to measure the thickness of the diaphragm or the second silicon substrate, for example, by a light interference measuring method while grinding the second silicon substrate. However, the amount of grinding the second silicon substrate varies. In addition, many concave portions, which are formed in the second substrate, vary in depth. Therefore, the thickness of the diaphragm varies significantly. Therefore, another problem is that it is difficult to finish the thickness of the diaphragm as desired with a good yield. Accordingly, the pressure measuring accuracy is reduced.
In another method of manufacturing a semiconductor pressure sensor as described in the publication above, the first alignment mark is formed to pass through the second silicon substrate including the concave portion and the diaphragm. The second silicon substrate has a thickness of at least 10 μm or more. Therefore, in order to form the first alignment mark, the second silicon substrate must be etched at least 10 μm or more from the first surface which is the bonding interface. This requires time and cost. Accordingly, the production cost is increased.
Furthermore, it is difficult to ensure the pattern accuracy of the alignment mark at the second surface of the second silicon substrate since the first alignment mark must be etched deeply to pass through the silicon substrate from the first surface of the second silicon substrate. Therefore, there is another problem of deterioration of the alignment accuracy. Accordingly, the pressure measuring accuracy is reduced.
In both of one and another methods of manufacturing a semiconductor device described in the publication above, gauge resistors are formed with reference to the second alignment mark registered based on the first alignment mark. Therefore, there is a problem that the registration accuracy of the gauge resistors is reduced as compared with when the gauge resistors are formed based on the first alignment mark without the second alignment mark. Accordingly, the pressure measuring accuracy is reduced.
Now, the method of etching a silicon substrate to form a concave portion, etc. mainly includes dry etching and wet etching. With either method, a minute amount of metal elements may contaminate a silicon substrate during etching or during cleaning after etching. In a semiconductor pressure sensor, a gauge resistor is formed by introducing an impurity in a silicon substrate by ion implantation, etc. The metal contamination may lead to variations in characteristics of the gauge resistor and reduction in reliability.
Supposing that the metal elements are unintentionally introduced into the silicon substrate in the step of forming the concave portion, the metal elements move in the silicon substrate when the silicon substrate is processed at high temperatures of 1000° C. or higher in the subsequent thermal processing step. Therefore, with such a configuration as the semiconductor pressure sensor described in the publication above in which a concave portion is formed in the second silicon substrate on which gauge resistors are to be formed, the movement of metal elements described above occurs in the thermal processing step carried out after bonding between the second silicon substrate and the first silicon substrate, so that the metal elements intrude into the inside of the gauge resistors. As a result, the characteristics of the gauge resistors vary, or the reliability is reduced. Accordingly, the pressure measuring accuracy is reduced.